Change 123 and Rappi Rangai at 1st glance can be seen as totally different but in fact are very alike. The genres are both ecchi and the artists are not trying to hide the fact. The two plots are both about 3 woman trying to protect a super weak boy. One difference to note is in Rappi the 3rd woman is a man cross dresser, and in Change 123, the 3 women are all 1 woman.
Besides characters, these two manga revolve around martial arts. Every character (besides the two weak boys) knows some top notch martial arts technique that enables her to annihilate any competition. So if your interested in martial arts and awesome ecchi manga, by all means get reading these two manga.
If you liked the "weak boy being protected (and loved) by several ass kicking girls" theme in Rappi Rangai, you might want to read Change 123. Not only that theme is present in Change 123, it has a better story and romance. Unfortunately same can't be said about the male lead and it should be noted that Change 123 contains a significant amount of ecchi (but it does have a story despite that).
Both Rappi Rangai and Sekirei are very similar harem series. They involve a weak male lead being protected by a harem of ass-kicking girls. Those girls not only consider him their master, but also are quite devoted to him. I don't think I need to mention that every single one of them is in love with the protagonist. Oddly enough the girls in both series seem to be based on the same archetypes (i.e. Musubi - Kagari). Although Sekirei has considerably more ecchi than Rappi Rangai, the story seems to be better there and the characters way more likable. If you liked one of those series, you might want to check the other one as well.
P.S. There is a gender-bender element in both series, but somehow it was far less annoying in Sekirei.